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Old 12-12-2012, 01:36 PM   #1227
DKShrimporium
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So, in an AMAZING string, TWO DAYS IN A ROW, DK sticks on topic. Wow, she's on a roll.

Yesterday, discussing the CJDS project we introduced this conceptual equation and talked about RESIDENCE TIME. Today, we talk about degassing rate.

The rough conceptual equation for degassing is:
RESIDENCE TIME x degassing rate x increase in temperature
= amount of degassing that occurs.


Gas will reach an equilibrium in solution, dissolving into the liquid. How much gas you can stuff into that liquid depends on a few things. Of course one is how soluble the gas is in the liquid. Another is the pressure of the system - more pressure forces more gas into the system. Another is temperature - cool temperatures allow more gas to dissolve into the liquid.

We can get all fancy and say it in some famous dude's words:

Henry's Law

The solubility of a gas in a liquid depends on temperature,
the partial pressure of the gas over the liquid,
the nature of the solvent
and the nature of the gas.

So let's review. DK's water emerges from her well - say, in winter months which are the most problematic and we solve our project problems based in part on the worst case scenario - at temperatures in the 50s F and with enough dissolved carbon dioxide to pull the pH down in the 5 or below range, fairly acidic.

Let's look at our graphic of the day - a nice fizzy soft drink, beading with condensation sweat, bubbling with carbonation, and you just know if you take a swig of it, you will feel the bite of carbonation. (Word geek alert: do you like DK's alliteration?)

Our soda illustrates this dissolved gas (carbon dioxide, even) in aqueous (that would be water-based) liquid solution.

What do we know about a soda?
  1. When you pop the can, you hear a pshhhhhhht sound, and if you don't, you know it's no good. That sound is the release of pressure in the can, that holds in MORE carbon dioxide gas in the soda, using pressure.
  2. You pour the contents of the can over a pile of ice, watching as you do the foam and bubbles, hearing the hiss of gas escaping solution and seeing this as microbubbles pop and produce tiny droplets, causing a sort of soda fog at the surface. This for most people is a compromise, because we want to drink it cold, but we know that we lose some fizz by pouring it out, pouring it over ice especially. It's not the cold of the ice, but rather the agitation of the liquid as it hits the solid matrix of ice that releases the carbon dixoide from solution, causing the foam and fog. We do it anyway, because there will still be enough gas left in the soda for a carbonation bite, even after we lose some from the ice agitation.
  3. If we put the soda back into the fridge, we know it will hold carbonation much longer than if we let it sit out hours or a day or two at room temperature. The colder temperature holds more gas in solution than warmer temperature, all other things equal.
  4. We also know not to shake a can or pour it high over the rocks, if we want to keep more carbonation. Once again, this is agitation, which releases gas from solution. In a closed 2 liter bottle (or can) that you have dropped on the floor, gas is released into the air space in the bottle/can, but the pressure inside the bottle (or can) if unopened rises from this, therefore eventually pushing more gas back into solution, and you end up back where you started. That is why bottled/canned soda can be dropped, heated, or whatever, and still hold the carbonation. The pressure will push the gas back into solution, as long as the bottle/can stays sealed.

OK, that's enough, for this post. Ponder on our soda picture, and the thought of dissolved CO2 in aqueous solution, and how agitation and temperature can drive the CO2 out of the solution.

The more agitation and temperature, the FASTER the degassing rate will be.

BTW - this explains why, if you are carbonating your planted tank with carbon dioxide (either DIY yeasty gas or pressurized from a cylinder), but then are using an HOB filter or canister spray bar on the surface, you are then agitating your gas right back out of solution, and shooting yourself in the foot, as the AIR above the water surface doesn't have as much CO2 as you are releasing from your water. On the other hand the AIR above the water surface has MORE oxygen than is down in the water, so the agitation is actually helping push oxygen into the water at the same time it is helping release the carbon dioxide.


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